The uncertainty principle of German scientist Werner Heisenberg states that the position and velocity of any object cannot both be measured exactly at the same time. In Simon Stephens’ much-celebrated play, “Heisenberg,” that theory is applied to people: two impressively dissimilar adults who meet awkwardly in a London train station and then begin a process of individual change, a change filled with immeasurables.

Now at the Mark Taper Forum, fresh from a much-celebrated Broadway run, the play proves very funny, intellectually engaging, and as rich in humanity as all of that implies.

Alex Priest, a stolid, elderly Irish butcher who lives alone in London, meets the significantly younger Georgie Burns when she impulsively kisses him on the back of the neck. Did she think he was someone else? We may never know, but her virtual stalking of him from that point forward, and her almost nonstop monologue on life, gradually shift Alex from his highly patterned, insulated isolation into a new view of the world around him.

The question, of course, is why she does this. What, in her constant speech, is the truth and what is fantasy? Is she a con artist, or genuinely fragile, or (as the British would put it) a bit mental? Does it matter, really, in Alex’s world?

This production has arrived in Los Angeles with the same two people who made it a sensation in New York. The chemistry between Denis Arndt and Mary-Louise Parker allows for the questions to fill the room, and yet not get in the way of watching two fascinating characters intertwine. Arndt’s Alex is delightfully underplayed, with small changes balancing well against the verbal and emotional abandon of Parker’s insecure Georgie.

The director, Mark Brokaw, who also created the New York original, has let these two extraordinary performances stand on their own. The performance is uniquely centered by set designer Mark Wendland in the Taper’s performance space, with only two easily moved tables and two chairs to provide any necessary physical needs. Thus, the performances are literally everything, a piece of brilliant stagecraft, as this is — indeed — all one needs.

Stephens’ script is delightful and wistful by turns, but never sentimental. There are moments of startling, delicious humor, and others of ponderable introspection. But most of all, in the hands of these two extraordinarily skilled actors, there is a particular kind of aching humanity, that delicate need for human connection that a modern social system makes easy to overlook.

“Heisenberg” is a fascinating exercise for many reasons. For someone who appreciates the things theater can do that no other medium does, the sheer sense of place and time expressed on a black block of a stage with minimal furniture is a treasure in itself. More than this, there is an elemental humanity at work in that space, not to mention two impressive examples of the actors’ art to savor.

For all these reasons and more, catch this one with these actors, in this setting, while you can.

Frances Baum Nicholson has been reporting on the Los Angeles area theater scene for more than 35 years. To read more of her reviews, go to www.stagestruckreview.com.